Diesel engines used on railroad locomotives commonly are equipped with turbochargers which are driven by the hot engine exhaust gases and serve to compress intake air. Since these exhaust gases may contain debris, such as pieces of broken engine valves, which can damage the turbine section of the turbocharger, prudence dictates inclusion of a debris separator in the piping between the engine exhaust manifold and the turbine inlet. Usually, the separator is an in-line unit containing a screen in the form of a flat perforated disc which extends across the flow path of the hot gases This type of separator has not proven entirely satisfactory, primarily because any foreign material entrained in the exhaust gas impacts directly upon the screen. In some instances, large metal pieces have broken through the screen and passed into and destroyed the turbine. Moreover, since the perforations of the screen must be relatively large (e.g., diameters of 3/16 - 1/4 inch) in order to keep to an acceptable level the backpressure imposed on the engine, these separators cannot remove small metal pieces and carbon particles, both of which may be detrimental to the turbine.
The object of this invention is to provide an improved separator for removing debris from the inlet flow to a hot gas turbine which is more effective and reliable than the screen type unit presently employed, yet entails a comparable expenditure and may be installed just as easily. According to the invention, debris removal is accomplished by the sequential use of centrifuging and screening operations. The separator comprises a casing containing an annular chamber in which the gases are swirled by stationary vanes to cause centrifugal separation of entrained material, a duct for extracting such material from an outer peripheral region of the chamber, and a screen of cylindrical form which defines the inner peripheral boundary of the annular chamber and is traversed by the gases after thay have passed through that chamber. The gas stream approaches the separator in the axial direction and is deflected radially outward to the annular chamber by a deflector which covers one end of the cylindrical screen. With this arrangement, much of the entrained debris is either deflected directly into the collection duct or is forced into the duct as a result of the swirling motion of the gases, and thus is removed without ever contacting the screen. The remaining debris, particularly the larger metal pieces, may be captured and withdrawn by the collection duct only after ricochetting within the annular chamber However, since none of the debris impinges directly upon the screen, and most of the kinetic energy of the ricochetting pieces is spent during deflections prior to any contact with the screen, the risk of impact damage to the screen is minimized. Furthermore, as a result of inclusion of the centrifugal separation step, the separator can remove small metal pieces and carbon particles, even though the perforations in its screen are sized to keep to an acceptable level the backpressure imposed on the engine.
Separators used in applications of the type under consideration here encounter fluctuating temperatures which may be as high as 900.degree. F to 1100.degree. F, so thermally induced stresses resulting from differential expansion and contraction of the parts can be a problem. Moreover, the unit is subjected to pulsating gas flow and considerable mechanical vibration. The preferred construction provided by the invention takes adequate account of these conditions. In the first place, the cylindrical screen used here is mounted within the casing in cantilever fashion. In addition, the turning vanes which swirl the gases are mounted freely in slots formed in the free end of the screen and are sized so that their outer tips are spaced radially inward from the surrounding region of the casing. These measures provide the flexibility needed to preclude build-up of crack-producing internal stresses, and thereby make the separator reliable under service conditions.
The casing of the preferred separator is provided with external flanges at its opposite ends and is adapted to be bolted in place between the engine exhaust and manifold and the turbine inlet piping in the same way as the screen separators now in service. The separator also includes several features which tend to minimize turbulence in the gas stream delivered to that inlet piping. One of these is the provision of a bleed passage which extends through the deflector and conveys into the interior of the cylindrical screen a very small portion of the hot gases entering the separator. This bleed stream precludes creation of a negative pressure area inside the screen, and thus tends to smooth the flow. Further flow-smoothing is effected by rounding the marginal edges of the perforations in the screen. Finally, the preferred separator provides a flow path of gradually decreasing cross section, so that the velocity of the gases increases as they pass from the exhaust manifold to the turbine inlet piping. Since the minimum flow area of the separator is at its outlet and is the same as the flow area of the existing turbine inlet piping, incorporation of the improved separator has no appreciable effect on the backpressure imposed on the engine.